Embodiments of the invention relate to the field of orthopedic surgery where joints (e.g., joints of the fingers and toes) may require surgical fusion. In particular, embodiments of the invention concern a set of devices that can prepare these joints for approximation in a ball-and-socket manner and then secure said joints to achieve fusion.
It is common in the field of orthopedic surgery to perform procedures on the small joints that make up the digits of the hands and feet. Specific disease patterns may affect the joints between phalanges (fingers or toes) causing pain and deformity. These interphalangeal joints, as they are known, can become destroyed from various processes. For example rheumatoid arthritis can cause rapid degeneration of the cartilage on either side of the joint due to repeated and continued inflammatory episodes. Another example is the contraction and subsequent subluxation seen in long standing “hammer toe” deformities, whereby the structures making up the joint permanently adapt to malposition. Once an interphalangeal joint is destroyed, or permanently altered, it is often the case that fusion of the joint is required. Arthrodesis, the fusion of two bones together, involves removing the cartilage or other material between bone ends, and permanently affixing the underlying cancellous bone material of adjacent bones together. This surgical procedure provides the benefit of pain and deformity relief, and prevention of return to the disease (inflammatory or malposition) state.
The optimal result of digital arthrodesis is permanent pain relief and deformity correction. In the course of surgical fusion the joint must be approached, disarticulated, prepared, and affixed. There are common approaches to the digit that are known by those skilled in the art that are essential to avoid neurovascular embarrassment and digit compromise. Generally this includes an incision on the dorsal surface of the affected digit, between the neurovascular bundles and above the tendinous structures. Disarticulation is the process of transection of the tendinous and ligamentous structures that maintain the joint. Sectioning of the collateral ligaments and joint capsule allows the surgeon entry into the joint. Once the joint is disarticulated, the ends of the bones, specifically their cartilaginous surfaces, become evident. It is necessary to then remove the cartilage from the ends of the adjacent bones to expose the underlying vascular or cancellous region. This is done through mechanical means, either by manual instrumentation or high-speed micro-saws. After the cartilage is removed, the bone ends are brought into approximation so the digit is aligned anatomically and affixed through one or more means. Finally, the soft tissue structures of the joint are reassembled. Fusion then occurs as the body forms new vascular channels between the apposed bone over the subsequent six to ten weeks.
Successful fusion of bone ends may depend on three factors: preparation, apposition and stabilization. Any intervening material must be sufficiently removed from the interface between the two bone surfaces without damaging the surface left behind. The ends of the bones may be in direct approximation such that new bone forming cells can bridge the gap. Additionally movement between the bones must be sufficiently below a threshold that delicate blood vessel and ultimately bone tissue formation can occur. This motion restriction may be maintained throughout the period of bone healing. To achieve this result attention must be paid to the manner of joint preparation and apposition.
Micro-surgical saws are high speed instruments designed to cut planar or flat surfaces. The instrument is commonly used in digital arthrodesis since it can rapidly remove the bone ends. There are some drawbacks to use of these machines. For example, excessive heat generation may occur when high speed instrumentation is used, causing bone cell death at the fusion interface. If enough of these cells are compromised the fusion can potentially fail, leading to the development of pseudoarthrosis, or ‘nonunion,’ as it is known. Additionally, these micro-saws are designed to produce planar (flat) cuts of bone ends. The intent is to then bring these flat surfaces together and have them approximate so that the ends are flush and completely coapted. The user is compelled to create co-planar bone cuts free-hand, since there are no jigs or cutting guides for this type of procedure. It is often difficult to freely make co-planar cuts perfectly such that the bone ends approximate exactly and the digital alignment is anatomic. As a result of imperfect joint apposition, the surgeon must remove additional bone to achieve a flush fit of the bone ends. This may lead to an excessively shortened digit.
Hand instrumentation can be of benefit since the slower process of joint preparation will likely not produce enough heat to cause bone cell death. A rongeur can adequately remove the cartilage from the joint. This instrument consists of two opposing jaws that clamp down on the cartilage and bone, grasping portions and detaching them. Alternatively a surgeon may use a bone nipper, a straight edged forcep designed to cut a planar surface while removing cartilage and bone material. These instruments can be used to remove very small portions of bone so as not to overly shorten a digit.
With repeated use a surgeon may become adept at using these instruments. However the use of hand instrumentation may lead to increased operative times since such procedures require a great deal of dexterity and repetitious movements. Additionally, these types of instruments are designed for multiple uses. However, as with all cutting edges, repeated use can dull the instrument, leading to imperfect cutting surfaces. The user then has to use additional force to remove bone, potentially causing damage to the cells left behind from the crushing effect of a blunt vice across the bone end.
Digital arthrodesis requires fixation to maintain bone approximation throughout the healing process. One example of fixation includes intramedullary stabilization using smooth steel wires, or ‘Kirschner wires,’ as they are known. These steel wires are introduced from within the arthrodesis site using a variable speed drill and protrude out of the end of the digit after the surgical procedure. They are meant to be removed after several weeks, upon completion of the fusion. They run down the central medullary canal of the bones, maintaining the apposition that the user creates while the bones fuse to one another. The wires stay in position because of friction forces from the bone in which it seats. Being perfectly smooth, often these wires will rotate throughout the healing process. This can create irritation at the skin interface, leading to pain, wire loosening, and possibly infection. Occasionally these wires may be inadvertently disrupted from a pulling force along their axis, such as seen with removing a sock or other dressing. This is because there is no stop measure to prevent axial distraction, such as a locking screw or other mechanism. Additionally, since these wires are perfectly smooth they generally cannot prevent rotation of one bone on another in the several weeks required for fusion.
One other method includes intraosseous loop fixation using fine gauge surgical steel suture. This method provides stability against bone rotation. Since the wires are buried inside the body the problem of inadvertent pulling of the fixation material is alleviated. However the placement of such small loops provides little stability against flexion forces and is often problematic since the procedure is lengthy, requires pre-drilling holes and sturdy bone substance to hold the suture. Consequently this method is less preferred over other methods of fixation.
Yet another method of small bone fixation is seen in devices that achieve bone stability yet are buried inside of the medullary canal of the small bones. These devices are intended to remain implanted permanently. They generally remain in place due to metal relaxation and deformation upon heating (memory-metal alloys), geometric fastening (arrow-head type device), or permanent adjoining (e.g., snap-together.) One skilled in the art of small bone surgery quickly realizes that often there is a need for removal of such devices, such as in cases of infection, inflammation, allergic reactions, malunion, and nonunions. Removal of these devices requires destruction of the surrounding bone to loosen the attachment of the implant. Since these devices cannot be retrieved without significant bone loss it becomes obvious that there is an advantage provided by fixation that is easily removed without additional bone removal.